loan offset account saves $47952, are my calculations wrong?

Just about to take out our first home loan, and i'm getting my head around all the loans and types of options such as offset accounts, fixed, variable and split loans etc.

As an exercise to see the benefits an offset account could provide, I ran some calculations in the attached excel spreadsheet, and it seems to indicate i'll save approx $47952 over the life of a $400,000 loan, making approx payments of $1400 a fortnight.

The problem is, i'm not convinced my calculations are correct and i'd really appreciate you guys casting an eye over my formulas to see what I am doing wrong.

Basically, I am calculating the daily interest rate (eg PA interest rate of 7.62/365 = 0.000208767) and multiplying that by the amount owing on the loan to figure out what the daily interest charge is (eg daily interest rate 0.000208767 * amount owing $400,000 = $83.50 interest on the first day of the loan etc)

In the offset example, I am slightly modifying it, to have the offset account balance reduce the amount owing for the purpose of calculating the daily interest (eg daily interest rate 0.000208767 * (amount owing $400,000 - offset account balance 5000) = $82.46 interest on the first day of the loan etc)

Anyhow if that all seems a bit confusing, have a look at the attached spreadsheet and it will hopefully be a bit clearer.

Note: linking to spreadsheet instead of attaching, as it is slightly over the allowed size limits.

Thanks Glebe, I updated it to reflect what you suggested about the initial $5K off the loan amount for the non offset account and changed the daily interest rate calculations on leap years.

I also accounted for ending the Offset Account option when there was 5K still owing on the loan, as you would simply transfer the balance of the offset account on it at that time to pay the remainder out.

So, I created a new sheet in the workbook to try a different scenario. What if I had an extra $500 a fortnight, I either put it into my non offset loan as larger payments, or I keep it in my offset account. The results again are surprising:

Borrowing $5k less or constantly keeping $5K in your offset a/c will naturally give you the same results as that is what an offset account does. It reduces the principle on which you have to pay interest... which is what you did by taking out a loan of $5K less.

The advantage of an offset account is that you can put all your income into it and for those days your interest will be lower. If you were to put all your income into the loan itself you would have to do redraws continuesly to pay for your living expenses. Some people even use their credit card to live on so that their cash can stay longer in the offset account. They naturally pay off the full amount on the card when it becomes due so as not to incur an even higher interest rate.